A wide variety of implantable medical devices are known and commercially available, including implantable cardiac pacemakers, cardioverters, and defibrillators, implantable neural stimulators, and implantable drug-administering devices. In the case of cardiac pacemakers, a variety of operational modes, or "pacing functions" have been defined. Pacemakers are generally characterized by which chambers of the heart they are capable of sensing, the chambers to which they deliver pacing stimuli, and their responses, if any, to sensed intrinsic cardiac activity. Some pacing functions specify that delivery of pacing stimuli is at fixed, regular intervals without regard to naturally occurring cardiac activity. Other pacing functions, however, specify that delivery of pacing stimuli is to be inhibited and/or triggered based upon sensed electrical cardiac activity in one or both of the chambers of the patient's heart. A so-called "VVI" pacemaker, for example, senses electrical cardiac activity in the ventricle of the patient's heart, and delivers pacing stimuli to the ventricle only in the absence of electrical signals indicative of natural ventricular contractions. A "DDD" pacemaker, on the other hand, senses electrical signals in both the atrium and ventricle of the heart, and delivers pacing stimuli only in the absence of appropriately timed natural a trial contractions, and ventricular pacing stimuli only in the absence of appropriately timed natural ventricular contractions. The delivery of each pacing stimuli by a DDD pacemaker is synchronized as much as possible with the patient's natural cardiac activity, as evidenced by prior sensed cardiac events.
Pacemakers are also known which respond to other types of physiologically-based signals, such as signals from sensors for measuring the pressure inside the patient's heart or for measuring the level of the patient's physical activity. In this way, the pacemaker's rate of delivery of pacing stimuli may be increased or decreased in correspondence with the patient's level of physical activity and demand for cardiac output.
The use of a pacemaker with a certain modality in a given patient is determined by the physician based upon several factors, including the nature and extent of patient's particular cardiac condition, and the patient's age and lifestyle. In addition, for a given pacing function, various operational parameters, such as pacing rate, A-V delay, pacing amplitude, sense amplifier sensitivity, and so on, must also be determined for each patient individually. Moreover, because a patient's cardiac condition is typically not static, (i.e. the patient can either show improvement or degradation in his condition), the indications for a particular pacing function and for particular parameter settings are subject to change over the years that the pacemaker may be implanted. In addition, a patient's cardiac condition at the time of implant may be difficult or impossible to diagnose, making it difficult for the implanting physician to determine what modality of pacemaker is indicated or what parameter values should be set.
The need to customize the pacing mode and pacing parameters for each patient at the time of implant, and the possibility that the mode and parameters might need to be altered over time has led to the development of various programmable pacemakers, in which the pacing mode and pacing parameters may be set and/or modified even after the pacemaker has been implanted in the patient. Today, nearly all pacemakers have at least some degree of post-implant programmable capability. Very sophisticated pacemakers may be programmed to operate in one of many different modes with a wide range of selectable parameter settings within each mode. Such programmable flexibility in implanted medical devices has proven to be very desirable, as evidenced by the widespread commercial success of these devices.
One disadvantage of pacemakers which are capable of being programmed into a variety of different pacing modes, with each mode having a wide range of programmable parameter selections, is that such pacemakers are typically larger, heavier, and more expensive than pacemakers operable in only one or a few modes, with relatively fewer programmable parameter options. Research and development costs, testing costs, manufacturing costs, battery consumption, and failure rates are all typically higher with programmable multi-mode devices than with simpler, single-mode devices. For these reasons, programmable multi-mode pacemakers have not entirely displaced single-mode devices in the market place. In addition, patients with readily diagnosed conditions may require, at least at the time of implant, only a simple, single-mode device, making the use of an sophisticated and expensive multi-mode device unnecessary and undesirable. However, these patients' conditions may nonetheless change over time, or new pacemaker features may be developed which are particularly well-suited to these patients. If a patient's pacemaker is not sufficiently programmable, making newly indicated or newly developed features available to that patient involves the removal of the old pacemaker, and the purchase and implantation of a new pacemaker. Replacement of a patient's pacemaker may also be required if his condition was initially mis-diagnosed.
Since the particular type of pacemaker that is required by a patient cannot be determined until that patient's cardiac condition is completely diagnosed, hospitals must maintain a large inventory of pacemakers of all modes, so that when the patients' conditions are diagnosed, the appropriate pacemakers are readily available. As pacemakers typically cost three to five thousand dollars, maintaining large inventories of these devices tends to increase hospital costs.
In most commercially available programmable medical devices, the selection for a programmable operational mode or selected values for operational parameters is stored in conventional random-access memory (RAM) within the device. One drawback of storing modes and parameter values in RAM is that conventional RAM devices are volatile memory devices; that is, information stored in a RAM is lost when power to the RAM is interrupted. Thus, most RAM-based programmable devices are provided with a set of "default" modes and parameters which define the operation of the device when it is first powered-up, or when power to the device is momentarily interrupted.
Although the selection of a particular operational mode is typically stored in RAM, the actual control instructions corresponding to the pacing function of each of the selectable modes in the pacemaker are usually stored in read-only memory (ROM) within the pacemaker. The selection stored in RAM, therefore, merely selects one of the pacing function definitions stored in ROM to be used for operational control of the pacemaker. Storing the pacing function in RAM is not generally considered acceptable, since a momentary interruption in power would completely disable the pacemaker, its instructions for executing a particular pacing mode having been lost due to the volatility of RAM. One disadvantage of storing the pacing function in ROM, however, is that the pacing function cannot be changed by means of external programming. If the pacing function is stored in ROM, only the selection of a particular pacing function, not the definition of that pacing function, can be changed via external programming.
In view of the factors mentioned in the foregoing discussion, the inventor has determined that it would be desirable to provide a pacemaker in which the pacing algorithm can be re-defined after implant by means of external, non-invasive programming, as is commonly used for selecting pacing modes and operational parameters.
It would therefore be desirable for a pacemaker to incorporate a memory device for storing the pacing function definition which combines the re-programmability of RAM with the non-volatility of ROM, so that the pacing function definition would not be lost in a momentary interruption of power to the device. If the memory were re-programmable like RAM, newly-developed pacing features could be implemented in pacemakers which were implanted before development of those features. Patients whose cardiac condition has changed since the time of implant of their pacemakers could have their pacing functions modified to accommodate their changed conditions.